1. Field of the Invention
This invention relates to a wafer stage assembly, a wafer table servo control system and to a method for operating the same in a photolithography process to manufacture semiconductor wafers. More particularly, this invention relates to the wafer stage assembly, wafer table servo control system and method of operation to increase focusing properties of the photolithography system.
2. Description of the Related Art
In manufacturing integrated circuits using photolithography, an energy beam, such as light, is transmitted through non-opaque portions of a pattern on a reticle, or photomask, through a projection exposure apparatus, and onto a wafer of specially-coated silicon or other semiconductor material. The uncovered portions of the coating that are exposed to light are cured. The uncured coating is then removed by an acid bath. Then, the layer of uncovered silicon is altered to produce one layer of the multi-layered integrated circuit. Conventional systems use visible and ultraviolet light for this process. Recently, however, visible and ultraviolet light have been replaced with electron, x-ray, and laser beams, which permit smaller and more intricate patterns.
As the miniaturization of a circuit pattern progresses, the focus depth of the projection exposure apparatus becomes very small, making it difficult to align accurately the overlay of circuit patterns of the multi-layered integrated circuit. As a result, a primary consideration for an overall design of the photolithography system includes building components of the system that achieve precision by maintaining small tolerances. Any vibration, distortion, or misalignment caused by internal, external or environmental disturbances must be kept at minimum. When these disturbances affect an individual part, the focusing properties of the photolithography system are collectively altered.
FIG. 1 illustrates a conventional exposure apparatus 21 having wafer stage assembly 100 being used in combination with a projection lens system 78 to manufacture semiconductor wafers 68. Wafer table 104 supports a semiconductor wafer 68, while a wafer stage 66 positions a semiconductor wafer 68 as wafer stage 66 is being accelerated by a force (not shown) generated in response to a wafer manufacturing control system (not shown). The wafer manufacturing control system is the central computerized control system executing the wafer manufacturing process.
In operation, exposure apparatus 21 transfers a pattern of an integrated circuit from reticle 80 onto semiconductor wafer 68. Exposure apparatus 21 is mounted to a base 82, i.e., the ground. Apparatus frame 72 is rigid and supports the components of exposure apparatus 21. Apparatus frame 72 supports reticle stage 76, wafer stage 66, lens assembly 78, and illumination system 74.
Different types of photolithographic devices, including a scanning type and a step-and-repeat type, have been used. In the scanning type photolithography system, illumination system 74 exposes the pattern from reticle 80 onto wafer 68 with reticle 80 and wafer 68 moving synchronously. Reticle stage 76 moves reticle 80 on a plane which is generally perpendicular to an optical axis of lens assembly 78, while wafer stage 66 moves wafer 68 on another plane generally perpendicular to the optical axis of lens assembly 78. Scanning of reticle 80 and wafer 68 occurs while reticle 80 and wafer 68 are moving synchronously.
Alternately, in the step-and-repeat type photolithography system, illumination system 74 exposes reticle 80 while reticle 80 and wafer 68 are stationary. Wafer 68 is in a constant position relative to reticle 80 and lens assembly 78 during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer 68 is consecutively moved by wafer stage 66 perpendicular to the optical axis of lens assembly 78 so that the next field of semiconductor wafer 68 is brought into position relative to lens assembly 78 and reticle 80 for exposure. Following this process, the images on reticle 80 are sequentially exposed onto the fields of wafer 68.
Regardless of the type of photolithography system is used, to focus accurately the image transferred from reticle 80 onto wafer 68, exposure apparatus 21 must align a position of an exposure point on wafer 68 with a position of a focal point of projection lens system 78. An auto-focus and auto-leveling (AF/AL) sensor (not shown) can determine the focal point position relative to the exposure point. However, the AF/AL sensor generally has several limitations, including, first, it only operates when wafer 68 is placed directly under projection lens assembly 78, and second, it requires significant signal processing causing a delay in generating an output. Furthermore, the AF/AL sensor measures the wafer surface, so the output of AF/AL sensor does not exactly measure the actual motion of wafer table 104. Hence, AF/AL sensor 114 is not sufficient for high bandwidth control of the wafer table 104.
In light of the foregoing, there is a need for an improved sensor for use in ombination with the AF/AL sensor, which can determine the position of the focal point relative to the exposure point with high bandwidth at all locations of wafer stage 66.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and consistent with the principles of the invention, as embodied and broadly described herein, one aspect of the invention is a stage assembly for use in combination with a projection optical assembly in a photolithography process for making a substrate. The stage assembly comprises a stage that positions the substrate, a stage base that supports the stage, and a table connected to the stage to support the substrate. The stage assembly also comprises a first sensor that determines a position of an exposure point on the table relative to the projection optical assembly, a second sensor that determines a position of a focal point of the projection optical assembly relative to the exposure point, and an actuator that moves the table so that the exposure point substantially coincides with the focal point.
A second aspect of the present invention is a method for determining a position of a table relative to a projection optical assembly in a semiconductor substrate manufacturing device. The table is supported by a stage and a stage base. The method comprises the steps of determining a position of the stage base relative to the projection optical assembly, determining a position of the stage relative to the stage base, and determining a position of the table relative to the stage.
A third aspect of the present invention is a table servo control system of a stage assembly having a stage base for supporting a stage and positioning a semiconductor substrate. A table is connected to the stage for supporting the substrate. The table servo control system comprises a first sensor controller to generate a first position signal of an exposure point on the table relative to a projection optical assembly, and a second sensor controller to generate a second position signal of a focal point of the projection optical assembly relative to the exposure point. The table servo control system also comprises a table controller to determine a correction force corresponding to the first and second position signals, the correction force being exerted on the table to bring the exposure point to substantially coincide with the focal point.
A fourth aspect of the present invention is a method for operating a table servo control system of a stage assembly. The method comprises the steps of creating a first loop to generate a first position signal of an exposure point on a table relative a projection optical system, creating a second loop to generate a second position signal of a focal point of the projection optical system relative to the exposure point, and exerting a correction force, in response to the first and second position signals, onto the table to bring the exposure point to substantially coincide with the focal point.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.